This invention relates to an FM-CW radar apparatus provided with:
a transmitter unit for the emission of frequency-modulated continuous carrier waves of bandwidth B; PA1 a receiver unit for the reception of echo signals reflected by a target, which originate from the carrier waves emitted by the transmitter unit, and for the generation of target-representing beat signals; PA1 a signal processing unit, to which the beat signals are applied, and which is equipped with: PA1 a control unit, which makes the output signals, generated by the first DFT processing unit, suitable for presentation on a display. PA1 a modulation sweep of the frequency-modulated carrier waves occupies a time of k.T seconds (k&gt;1, k .epsilon.R); PA1 a DFT sample period of the first DFT processing unit occupies a time of T seconds, so that a modulation sweep comprises more than one DFT sample period; PA1 the FM-CW radar apparatus is provided with a second DFT processing unit, to which is applied at least part of the first DFT processing unit's output signals, associated with one modulation sweep, and an output signal of which, containing target range information of a higher resolution than that generated by the first DFT processing unit, is applied to the control unit.
an A/D conversion unit for the sampling and digitisation of the beat signals; PA2 a first DFT processing unit, to which the sampled and digitised beat signals are applied with the object of obtaining at least range information about the target;
The advantage of such a radar apparatus is that the emitted carrier waves are difficult to detect. This is due to the electromagnetic energy to be emitted being given off spread in time and distributed over a large bandwidth. In consequence of the spread in time the power is low, and in consequence of the relatively large bandwidth B the energy per frequency unit .DELTA.f is small.
Radar apparatuses of the pulsed type exhibit the exact opposite: due to the emission of pulses, the electromagnetic energy is given off compressed in time. This implies that the power is high.
For civilian applications, an FM-CW radar apparatus has the advantage, with respect to a pulse radar apparatus, that the modulated carrier waves cause little interference to other equipment, such as communication equipment.
For military applications an additional advantage is that, for an adversary, it is virtually impossible to detect the modulated carrier waves of an FM-CW radar apparatus. This is because the energy per frequency unit .DELTA.f is so small that it is lost in noise.
It is known that the range resolution .DELTA.R of an FM-CW radar apparatus is substantially equal to C/2B, C representing the speed of light and B the bandwidth. If an N-point DFT processing unit is employed, the maximum detectable range R.sub.max =CN/4B.
For quite some time the need has been felt for an FM-CW radar apparatus of longer range. It is known that doubling the maximum range (2R.sub.max) can be achieved by halving the bandwidth B. It is also known that the maximum range of an FM-CW radar apparatus can be adjusted to R'.sub.max =k.R.sub.max (k.gtoreq.1; k .epsilon.R) by making the bandwidth equal to B/k. Thus, existing equipment (with the same N-point DFT processing unit) enables detection of a target at a larger range.
However, since the bandwidth is reduced by a factor k, the detectability of the FM-CW radar apparatus is increased by the same factor. This implies that the advantage of an FM-CW radar apparatus, viz. the virtual undetectability of the emitted carrier waves, is lost. An additional disadvantage is that the range resolution decreases with a factor of k.
The invention fully solves both problems, yielding an FM-CW radar apparatus of increased range, without deterioration of the detectability and the range resolution.